One of the objects of this invention is to minimize the voltage drop across electric low-voltage fuses, and thus to provide a family of electric low voltage fuses that minimize heat losses and thus conserve electric energy.
To this end the current path of fuses according to this invention comprises a relatively thin center portion having serially arranged perforations, and two terminals of which at least one is in the form of a single tab having a thickness that is considerably larger than the thickness of the fusible element and thus reduces the voltage drop across the fuse. That tab is bent across one of the rims of the casing of the fuse to the outer surface thereof where it is conductively engaged by, or conductively connected with, one of the terminal caps of the fuse. The tab consists of copper, or an alloy thereof, that has a relatively small resistivity, and the center portion of the fusible element is either of silver or of copper, i.e. of a current-limiting metal.
Fuses as disclosed e.g. in U.S. Pat. No. 3,240,905 to F. J. Kozacka; Mar. 15, 1966 for LOW VOLTAGE FUSE HAVING A CASING OF CELLULOSIC MATERIAL AND AN ARC-QUENCHING FILLER OF QUARTZ SAND have a perforated fusible element of copper which may be substituted, if desired, by one of copper, and terminals in the form of blade contacts which are of copper. These terminals are not adapted to be used, nor do they suggest to be used, in the way of the tabs of the present invention, i.e. to be bent across one of the rims of the casing of the fuse to the outer surface thereof, and there conductively connected with the terminal caps of the latter.
As will be explained below in detail, the thickness of the tabs according to this invention must have a critical ratio to the thickness of the fusible element. This ratio depends primarily on the maximum degree of reduction of voltage drop to be achieved, on whether or not the requirements of the Underwriters Laboratories Inc. for Standard Class R fuses are fully met, or only approximately met, and to some extent also on the geometry of the fusible element or its heat dissipation in the surrounding pulverulent arc-quenching filler.
Another object of the invention is to provide electric fuses with a tab, or a pair of tabs, which has, or which have, a minimal effect on heat dissipation. This will be explained below in greater detail.
Still another object of this invention is to provide Underwriters Laboratories Inc. Standard Class R fuses which are inexpensive to manufacture and inexpensive to operate on account of their reduced voltage drop.
A further object of this invention is to provide cool running fuses which is likewise due to their relatively small voltage drop.
Still another object of the invention is to overcompensate by the increased thickness of the terminal tab, or tabs, or copper for the larger resistivity of copper relative to that of silver.
In this context silver means also alloys of silver having substantially the physical properties, or pre-arcing I.sup.2 .multidot.t values, of what is referred to in the trade as pure silver. Similarly copper means also alloys of copper having substantially the same physical properties, or pre-arcing I.sup.2 .multidot.t values, of what is referred to in the trade as electrolytic copper.